The invention pertains generally to the field of filters useful in the 2 to 18 Gigahertz (hereafter GHz) range, and, more particularly, to tunable band reject and bandpass filters using yttrium-iron-garnet (hereafter YIG) ferrimagnetic tuning spheres.
In high frequency receiver systems operating in electrically noisy or hostile environments, it is often useful to have "wide-open" receiver systems which can pick up signals throughout a large band of frequencies. Frequently in such wide-open receiver systems, undesired signals can appear in the band. The unwanted signals can create an annoying interference or render the receiver system inoperable by saturating the input amplifier stages thereby blinding the system to desired signals.
To reduce or eliminate the effects of these unwanted signals, a class of tunable high frequency notch filters grew up. The purpose of this type of filter is to reject energy in an electronically tunable band of frequencies which is typically 25-50 megahertz (hereafter MHz) wide. This reject or stop band has a center frequency which is ideally tunable throughout the range of frequencies for which the receiver system is designed.
Designing a tunable notch filter which has a deep, narrow notch that can be tuned over a broad bandwidth is a difficult challenge. Approaches which were tried and which failed in the 1978 time period were mechanically tuned notch filters and switched, fixed-frequency filters. The mechanical filters failed because of poor reliability, limited tuning range and high costs. The banks of switched, fixed-frequency filters failed for similar reasons.
In response to these shortcomings, a class of YIG tuned notch filters were developed. All YIG notch filters are basically comprised of a transmission line structure which carries the RF signal to be applied to the receiver input and several YIG spheres which are coupled to the transmission line by RF coupling loops at points along the length. Although many different species of these YIG filters exist, they all have certain common characteristics. Typically the YIG spheres are coupled to the transmission line by half or full loops which create radio frequency (RF) magnetic fields which engulf the spheres. The YIG spheres act like tuned circuits, i.e., an inductor connected to a capacitor. Tuned circuits have a resonance frequency at which the impedance of the tuned circuit changes drastically from the impedance at frequencies other than the resonance frequency. A tuned circuit with the inductor and capacitor connected in parallel has a combined impedance which peaks, i.e., reaches its highest value, at the resonance frequency. A series tuned circuit has the inductor connected in series with the capacitor. In these types of tuned circuits, the combined impedance reaches a minimum at the resonance frequency.
Typically, the YIG spheres of a tunable notch or band reject filter are used as both parallel tuned circuits in "series" with the transmission line and as series tuned circuits coupled in "shunt" to the transmission line. This is done in prior art YIG filters by connecting the RF coupling loops together by one-quarter wavelength transmission line sections usually in the form of 50-ohm stripline formed on a substrate. These one-quarter wavelength sections are used as impedance transformers to effectively convert the electrical equivalent circuit of every other YIG sphere from a series-connected, parallel tuned circuit to a shunt-connected, series tuned circuit. The overall electrical equivalent circuit for the resulting YIG tuned notch filter is then a series of series-connected parallel tuned circuits separated by a number of shunt-connected series tuned circuits.
The resonance frequencies of all the tuned circuits in the above-described structure are all at the same frequency because all the YIG spheres are typically subjected to the same D.C. bias tuning magnetic field. That is, in this prior art structure, all the YIG spheres are typically placed in the same magnetic gap between two magnetic pole pieces. These magnetic pole pieces typically have coils of wire wrapped around them that carry a D.C. bias current. This current creates a steady magnetic field which is designed to be uniform throughout the gap between the magnetic pole pieces. This magnetic field envelopes the YIG spheres and magnetically biases each YIG sphere to have the same electrical resonance frequency. By changing the amount of D.C. bias current flowing through the magnet coils, the resonance frequencies of all the YIG spheres can be simultaneously tuned to the same, selected center frequency where the notch is desired.
The effect this has on the wide spectrum of RF frequencies passing through the YIG, notch filter (hereafter the passband) is to "cut a notch" in the passband. This notch is caused mainly by a high mismatch at the resonance frequency, i.e., the notch center frequency of the series-connected, parallel tuned circuit which reflects RF by virtue of being an "open circuit" in which no energy can pass and the low impedance to ground of the shunt-connected, series tuned circuit which reflects RF by virtue of being a short circuit through which no energy passes. That is, for a relatively small (typically 25-50 MHz wide) notch band, centered about the resonant frequency of the YIG spheres, the YIG spheres attenuate RF signals such that substantially less RF energy having frequencies in the notch band leave the YIG filter than entered it.
FIG. 1 is a block diagram of a typical prior art YIG stopband or notch filter application and FIG. 2 is a graph of YIG notch filter performance which illustrates the above-described phenomenon. In FIG. 1, an antenna 10 captures RF energy over a wide band of frequencies, say, for example, 2-18 GHz. This RF energy is guided to the input 11 of a YIG notch filter 12 which has a tunable center frequency for its stopband. This tunable notch or stopband is shown at 14 in FIG. 2. The vertical axis in FIG. 2 represents the attenuation loss imposed by the YIG filter on RF energy having the frequencies covered by the horizontal axis. The downward direction along the vertical axis represents increasing attenuation. RF energy in the stopband 14 at input 11 is reflected back toward the antenna or absorbed by the losses in the filter.
The resulting filtered RF energy, now lacking the RF signals that were within the stopband 14, is then guided to the input of a wide-open type receiver system symbolized by block 16. The receiver then outputs appropriate signals for use by a user via a user interface 18.
In FIG. 2, the frequency range from point 20 to point 22 represents the range of frequencies called the passband which the receiver 16 can detect. Assume now that an interfering signal at frequency F is detected and it is desirable to remove it. In such a case, a tuning command may be issued on line 24 either from the receiver 16 (such receivers typically have alarm circuits which help detect interfering signals) or from the user interface 18. This tuning signal will alter the level of D.C. bias current flowing through the magnet coils in the YIG filter 12 until the center frequency F.sub.c of the notch 14 is shifted so as to match the frequency (or center frequency) F or the interfering signal. The attenuation in the stopband of notch 14 then removes the interfering signal from the spectrum of RF energy which appears at the input of the receiver.
FIG. 3 represents the electrical equivalent circuit of the YIG notch filter. The equivalent circuit of FIG. 3 represents a three sphere YIG notch filter. Assume that the spheres are numbered 1 through 3 in order from the RF input 26 to the RF output 28. YIG spheres 1 and 3 have the series-connected, parallel tuned circuit equivalent circuit shown generally at 30 and 32, respectively. YIG spheres 1 and 3 are each connected to YIG sphere 2 in prior art YIG filters by one-quarter wavelength transmission line sections symbolized by lines 34 and 36. These sections convert the equivalent circuit of sphere 2, also shown as a parallel-tuned circuit, to a shunt-connected, series tuned circuit 38. If the YIG spheres were to be removed, the tuned circuits would disappear from the equivalent circuit of FIG. 3, and the resulting equivalent circuit would be a straight through transmission line coupling the input 26 to the output 28.
Typically, the antenna 10 and the input of the receiver 16 in FIG. 1 are designed to have a characteristic impedance Z.sub.0 of 50 ohms, which is an industry compatibility standard. It is axiomatic in electrical engineering that to maximize the efficiency of power transfer from the antenna to the receiver, one must match the output impedance of the antenna to the input impedance of the receiver. Likewise, where a YIG filter is placed between the input of the receiver and the antenna, to maximize the efficiency of power transfer from the antenna to the receiver, it is necessary that the YIG filter have an input impedance that matches the output impedance of the antenna and an output impedance which matches the input impedance of the receiver. If the input impedance of the YIG filter at 11 in FIG. 1, does not match the output impedance of the antenna (or the intervening transmission line), RF power is reflected back toward the antenna thereby creating a voltage standing wave in transmission line 40. The voltage standing wave ratio or VSWR is a measure of the degree of impedance mismatch.
The graph of FIG. 2, shows several small notches at 42 and 44. These notches are undesirable side effects called spurious responses or modes which are intrinsic to the use of YIG spheres. The most troublesome spurious response is the (2,2,0) Walker mode (also referred to herein as the 220 mode or 220 spurious response). It is desirable to eliminate these spurious responses.
It is also important for the YIG filter to attenuate the passband RF energy at frequencies on either side of the notch as little as possible. Any undesired attenuation is called insertion loss.
Thus, a good YIG notch filter will have the following characteristics: 1) a good impedance match with the devices coupled to its input and output, i.e., low VSWR, 2) low insertion loss, 3) minimal spurious responses, 4) deep notches of 25 to 50 MHz bandwidth, with a high Q factor, 5) agile, fast notch tuning throughout the passband, 6) predictable stopband characteristics, 7) a wide passband wherein the above stated characteristics remain within an acceptable range throughout the passband.
Workers in the art have been trying to achieve all of the above characteristics in a single design that covers a 2-18 GHz passband for many years. Several major problems exist which make achievement of all the above objectives over a passband of 2-18 GHz very difficult.
First, the physical length of the one-quarter wavelength impedance inverter is only correct at one frequency in the passband. At other frequencies, the inverters are not one-quarter wavelength long so the desired 90.degree. phase shift of a true one-quarter wavelength is not achieved exactly. Further, the one-quarter wavelength striplines impose increased insertion loss because of their non-air dielectrics.
Second, the bandwidth of the stopband or notch is dependent upon the loop coupling, with greater coupling giving a wider notch which is considered desirable by workers skilled in this art. Generally, full loop coupling where the YIG spheres are surrounded by full loops as opposed to half loops gives a wider notch.
Full loop coupling also has two other advantages which make its use throughout the notch tuning range highly desirable. First, full loop coupling is not nearly as sensitive to manufacturing errors in positioning of the sphere within the coupling loop. Second, full loop coupling does not generate as many spurious responses. With half loop coupling, a small positioning error in positioning of the YIG sphere within the half loop leads to a substantial change in the overall coupling. Full loops do not suffer from this problem since a positioning error leads to tighter coupling between the YIG sphere and one portion of the loop and looser coupling between the sphere and another part of the loop opposite the part of the loop with tighter coupling. The overall coupling remains approximately the same as it would have been absent the positioning error. Fewer spurious responses result from full loop coupling, because this type of coupling is more symmetrical than half-loop coupling. Spurious responses are generally caused by asymmetrical coupling.
The problem with full loop coupling is that the full loops have more inductance than half loops and this extra inductance is too high to be effectively compensated by shunt capacitance at the high end of the passband. The inductance of the coupling loops periodically loads down the transmission line throughout structure of the YIG notch filter at the locations of the YIG spheres and therefore affects the passband by dominating the characteristic impedance of the transmission line from the YIG filter RF input to its RF output, The characteristic impedance of a transmission line is: EQU Z.sub.0 =.sqroot.LC
where,
Z.sub.0 =the characteristic impedance per unit length, i.e., per section PA1 L=the inductance per unit length PA1 C=the capacitance per unit length.
Inductive reactance varies directly with frequency. Thus the reactance of a coil of wire increases as the RF frequency feeding the coil increases. This is the reason all attempts in the prior art to use full loop coupling at the 18 GHz end of the passband have failed until now. At these high frequencies, the inductance of the full loop coupling coils was so high that it was impossible to add enough capacitance per section to hold the characteristic impedance down to the industry standard 50 ohms over a wide range of frequencies. The impedance mismatch resulted in part of the desired RF energy in the passband outside the notch being reflected back toward the antenna. This caused an unacceptably high VSWR value.
As a result, in the prior art, different filter structures were used for different frequency ranges. In the early years of YIG filter design, YIG notch filters with full loop coupling were designed to operate in the range from 2-4 GHz. Another design was used for the range from 4-8 GHz and another design was used for the range from 8-18 GHz generally employing only half loop RF coupling. Generally, the length of the one-quarter wavelength sections was made smaller for the higher frequency designs and the coupling was changed from full loop at the low frequencies to half loop at the higher frequencies. It was necessary to shorten the one-quarter wavelength sections at the higher frequency to avoid the distortion in the notch shape which would otherwise result. The switch to half loop coupling at the high frequencies was necessary to keep the input and output impedances of the YIG filter near 50 ohms, but it also resulted in greater spurious responses.
Eventually, workers in the art determined that since the use of more YIG spheres gave a deeper notch, it was possible to stretch the design of a YIG filter such that one or possibly two designs could be used to cover the whole passband from 2-18 GHz simply by using more spheres. Thus, even though the performance steadily degraded at the lower frequencies, useable performance levels could still be obtained. That is, performance which was adequate to meet the specifications of customers was obtainable since more than enough notch attenuation and stopband bandwidth at the high frequencies was available and the degradation at lower frequencies was not enough to take the performance out of the acceptable range. However, none of these designs in the prior art were capable of using full loop coupling at the high frequency end of the passband, because of VSWR problems caused by the increasing inductive reactance of the full loops at high frequencies. As a result, to date no designer of YIG filters has been able to use full loop coupling to the YIG spheres in the high end of the passband near 18 GHz.
A short history of the various specific approaches that have been tried in the prior art is in order so as to better frame the subject and provide greater appreciation for the differences over the prior art of the approach taught herein according to the teachings of the invention. In 1978, the state of YIG filter design was generally as taught by W. J. Keane in his article "Narrow-Band YIG Filters Aid Wide-Open Receivers", published in Microwaves in September 1978 at page. 50 which is hereby incorporated by reference. In that article, basic YIG filter designs are discussed as are the various requirements for a good YIG filter design. The concepts of shunt capacitors to control Z.sub.0 and VSWR considerations and insertion loss and the desirability of linear phase characteristics throughout the passband are discussed. Also discussed are the relationship between spurious responses and tighter coupling. The tradeoffs between passband and stopband performance are discussed. The distortion in the shape of the notch as it is tuned over the passband caused by non-optimum resonator spacing is also discussed. This article states that the passband design fixes the stopband performance since the size, shape and spacing of the coupling loops that are matched in the transmission line determine the amount of coupling bandwidth achievable with a given size and number of spherical YIG resonators. The article goes on to state that the generation of spurious modes due to nonlinear RF fields in the YIG resonators ultimately limits the minimum sphere-to-loop spacing. This statement assumes that half loop coupling is used, and there is no suggestion that full loop coupling can be used. In fact, this article suggests that since passband performance is paramount, full loop coupling cannot be used at high frequencies since it would cause high VSWR and large insertion losses which would be very disadvantageous. The maximum tuning range without excessive notch shape degradation is suggested to be two octaves.
The state of the art of prior art, full loop YIG notch filter design prior to the invention is probably best understood by referring jointly to FIGS. 4A, 4B and 4C. These figures show a 4-sphere YIG notch filter design using full coupling loops and designed for use in a 2-6 GHz passband. Although frequently 6 to 8 YIG spheres would be used in conventional designs, only 4 spheres are shown here for simplicity. FIG. 4A shows a plan view of the YIG notch filter while FIG. 4B shows an elevation view and FIG. 4C shows the filter in pseudo-schematic form. In this prior art design, the YIG spheres 46, 48, 50 and 52 are supported in a magnet air gap 54 between two circular cross section electromagnet pole pieces 56 and 58 by beryllium oxide support rods 60, 62, 64 and 66 which are anchored in heater blocks (not shown). The pole pieces have wound around them D.C. bias electromagnet coils 68 by which a D.C. magnetic field is created in air gap 54. In the prior art designs symbolized by FIGS. 4A through 4C, the pole pieces 56 and 58 were separated by an air gap which was approximately 0.060 inches across. By changing the intensity of the D.C. magnetic field in this air gap, the resonance frequency of all of the YIG spheres could be simultaneously tuned thereby altering the center frequency of the band reject notch.
The YIG spheres used in prior art designs generally used spheres which were 0.015 to 0.030 inches in diameter. In addition, between each sphere, a 50 ohm microstrip or strip line transmission line was used as an impedance inverter. These microstrip or stripline impedance inverters are symbolized in FIG. 4A by the line between solder joints 71 and 73, the line between solder joints 75 and 77, and by the line between solder joints 79 and 81. The 50 ohm transmission line impedance inverters were fabricated on an insulating substrate (not air) and were designed to be 1/4 wavelength at some frequency in the desired passband.
Because the impedance of the full coupling loops became very high at 18 GHz, it was impossible in the prior art designs symbolized by FIGS. 4A-4C to match the overall impedance of the coupling structure to the 50 ohm line at the RF input 78 and the RF output 83. The resulting impedance mismatch caused major VSWR problems and higher insertion loss. For these reasons, the full loop designs were not used at 18 GHz. At these higher frequencies, 1/2 loop coupling straps were used in the prior art as symbolized by FIG. 5 instead of insulated wire. The 1/2 loops cut down on the available coupling, and caused more spurious modes. Further, they were sensitive to manufacturing errors in placement of the spheres in the centers of their cavities. However, the half loops were easier to match to 50 ohms, because their impedance was less at 18 GHz, and they had proportionally more surface area to capacitively couple to the cavity walls to provide the shunt capacitance C in equation (1) above. Additional capacitive coupling between the coupling loops and the ground plane was provided by grounded, adjustable shim plates of which plates 88 and 90 were typical. These shim plates generally were used to cover the tops of the cavities formed in a metal block like block 89 in FIG. 4B suspended in the air gap 54 which contained the YIG spheres. Capacitive coupling to adjust impedance and VSWR was altered by deforming the shims to push them closer to or further away from the RF coupling loops. Also, the cavities in the prior art designs, of which cavity 91 in FIG. 4B is typical, were spaced much further apart in the prior art designs than the spacing according to the teachings of the invention and were arranged in a circle. Only the cavities of the spheres 46 and 52 of FIG. 4A are visible in the cross section of FIG. 4B with the cavities of the spheres 48 and 50 obscured behind.
The YIG notch filter according to the teachings of the invention is aligned to match all center frequencies of the YIG spheres as closely as possible by rotating all the spheres in their cavities until best performance is achieved.
The structures of FIG. 4A and 5 are the structures upon which the article by W. J. Keane cited above was based. For the high frequency part of the desired passband from 6-18 GHz (typically), the prior art half loop structure FIG. 5 would be used with all other structural details being the same except that sometimes coupling straps were used instead of wires. Since the half loops 80, 82, 84 and 86 have substantially less inductance at the high frequency end of the passband, it is possible to add enough shunt capacitance via shim plates 88, 90 92, 94, 96, 98, 100 and 102 to keep Z.sub.0 down to close enough to 50 ohms to result in an acceptable VSWR.
In December of 1979, U.S. Pat. No. 4,179,674 (hereafter the '674 patent) to Keane et al. issued which is hereby incorporated by reference. This patent taught a RF coupling structure for non-reciprocal coupling using half loops and a cover over the YIG spheres to increase the shunt capacitance between the coupling loop and the groundplane. This shunt capacitor produced a phase shift which caused circular or elliptical polarization and was thought to increase the Q from the 100 to 500 values previously achieved to the 20,000 range for both the VHF and microwave ranges. The '674 patent also teaches dividing the RF coupling loop up into two coupling loops which couple to the YIG sphere in a spatially orthogonal fashion with phase orthogonality produced by a divider circuit to produce circular polarization. The '674 patent teaches linear polarization as yielding an absorptive filter. This patent also teaches offsetting the YIG spheres from the plane of the RF coupling loops to generate circular polarization and coupling a shunt capacitor to one of the electrical transmission lines to introduce a 90.degree. electrical phase shift and circular polarization. The '674 patent teaches that by offsetting the YIG sphere and using circular polarization reduces spurious responses. The '674 patent also teaches that offsetting without circular polarization has the opposite effect in producing more spurious responses. The patent also teaches that the sense of the polarization is important in eliminating spurious responses. This depends upon the sense of the static magnetic field, and the side of the loop to which the sphere is offset. A single line YIG sphere offset and a capacitor coupled to the middle of the loop is also taught in FIG. 10 to achieve single line. An adjustable shim casing to vary the shunt capacitance is also taught.
In August 1980, U.S. Pat. No. 4,216,447 to Keane et al. was issued which contained the same teachings as U.S. Pat. No. 4,179,674, but which claims different subject matter.
In January of 1981, U.S. Pat. No. 4,247,837 issued to Mezak, et al. (hereafter the '837 patent) which is hereby incorporated by reference. This patent teaches using multiple conductors in the coupling loop to increase the coupling to the YIG spheres. The multiple conductors of the coupling loop are separated by at least one diameter and are taught to be a superior approach in attempting to get notch bandwidth greater than 35 MHz. Prior approaches to increase the notch bandwidth included bringing the coupling loop closer to the sphere and using a lower inductance conductive strap as opposed to a single wire loop. Both of those prior approaches are taught to have increased "crossing" and "tracking" spurious responses. "Tracking" spurious responses are unwanted spurious mode notches that move with the center frequency of the desired notch. Crossing spurious responses move at rates different than the center frequency of the desired notch. The '837 patent teaches the conventional wisdom that prior attempts to increase the notch bandwidth by increasing the turns in the coupling loop did not work and, therefore, teaches away from the invention. This was because of the increased series inductance in the line from the input to the output which lowered the frequency of the high frequency cutoff end of the passband above which the filter was useless. The '837 patent also teaches that prior attempts to solve this problem included use of a strap and multiple closely spaced or touching wires in the coupling loop both of which approaches have failed because of increased spurious responses.
Significantly, the '837 patent, at Col. 2, lines 2-8 and Col. 6, lines 3-19, teaches that it is disadvantageous to increase the number of turns of the coupling loops because it increases the series inductance in the line between the input and output ports. The '837 patent teaches that use of multiple, separated conductors works better than large diameter wires or straps because less spurious modes are created. The multiple conductors of the '837 patent are used in RF coupling loops only and are not used in the transmission line segments between coupling loops.
In 1986, Watkins-Johnson announced, in the April issue of Microwave Journal, an 8-stage, YIG-tuned notch filter with a 60 MHz notch bandwidth and a 6-12 GHz passband. This filter had tracking spurious responses of 4 db (maximum) and a VSWR of 2:1 (maximum). Passband insertion loss was 1 db (maximum). Subsequently, Watkins-Johnson announced, by data sheet, a series of 8-stage, YIG-tuned notch filters that cover various segments of the passband from 0.5 to 26 GHz. Notch bandwidth varied from 5 to 35 MHz with 40 db of rejection and VSWR values ranged from 1.5:1 to 2:1 and insertion loss values ranged from 1 to 2 db. Tracking spurious modes were 4 db maximum.
In April 1989, the then existing state of the prior art in YIG-tuned notch filter design was summarized by W. J. Keane in a design note that was widely circulated to customers for YIG filters. This design note discussed the relative merits of 4-sphere vs. 7-sphere YIG filters and the theoretical tradeoff between band-reject and passband performance. This design note is included herewith as Appendix A.
In this design note, the effect of the fraction of coupling turns of the RF coupling loop on the notch bandwidth is discussed at page 11. There, four different approaches in the prior art are identified for designing the passband for a YIG notch filter. The first approach is to design a loop whose impedance is nominally 50 ohms over its entire length. In this approach, the YIG spheres are placed in cavities with the coupling loops in the cavities positioned between the cavity walls and the sphere. The coupling loops are connected by impedance inverters in the form of 90.degree. lengths of stripline transmission line. The disadvantages of this approach is that it needs a sizeable area to accommodate all the cavities and striplines. This makes the magnet gap large in area and renders it more difficult to achieve equal magnetic flux intensity for all spheres. Large gaps also make rapid tuning of the notch center frequency more difficult since large amounts of magnetic flux need to be changed in flux density level to tune the center frequency. It is believed that Watkins-Johnson uses this approach.
The second prior art approach discussed in the Keane design note is a stripline circuit where the loop inductance is matched using distributed capacitance on both sides of the cavity. However, for large fractional coupling loops, it is difficult to achieve sufficient capacitance to make the match at the high frequency end of the passband. Another disadvantage of this second approach is the fact that the cavities and impedance inverters are not totally shielded, making it difficult to provide isolation between the input and output. This approach could be used at lower frequencies to match a single or multiple turn coupling loop.
The third approach is an iterative matching procedure consisting of a single transmission wire that is periodically loaded by the RF cavity walls. One advantage of this approach is high fractional coupling factor including full or multiple full turn loops. This increases the notch bandwidth and reduces coupling into magnetostatic modes such as the 210 mode. Also, nonreciprocal coupling can be used to suppress several other modes. This approach requires less area for the RF magnet gap and the RF circuit. The insertion loss and VSWR for this approach can be very good depending upon the passband. However, at the time this third approach was developed, the use of the full loop coupling was contemplated only for low frequencies where it was possible to match the line impedance using the casing. The Ferretec design criteria note acknowledges this conventional wisdom at page 15 indicating that designs of the day were generally intended for either a low band of 2-6 GHz or a high band from 6-18 GHz.
The fourth approach consisted of designing a low pass filter structure which combined distributed loop coupling and discrete inter-loop capacitors. This approach is practical for broadband designs only at low frequencies. However, the fractional coupling factor n cannot be as large as with the iterative matching approach. This approach has been used primarily in the low passband range.
In February 1985, the assignee of the present invention made and sold to Watkins-Johnson a YIG notch filter according to the third approach but using a pair of wires to form both the coupling loops and the inter-connecting transmission line segments rather than a single wire. Four YIG spheres and 1/2 coupling loops were used. Band reject performance was unsatisfactory from 2-16 GHz until the twin wires were soldered together at the midpoint between the spheres 2 and 3. This changed the coupling from "unusual" at 10 and 20 GHz to "normal" at 10 GHz and unusual at 20 GHz. Subsequently, three solder joints were used, one between each loop at the midpoints of the connecting segments. This caused the structure to have normal coupling from 4-26 GHz and behave like a single loop. The notch appeared most symmetric at 14.5 GHz.
Changing the wire diameter from 0.003 inches to 0.0075 inches with three solder joints improved the performance further.
A variant was then tried with the outer sphere 1 and 4 coupled by half loops and the inner spheres coupled by full loops. In this embodiment, the cavity diameter was 0.060 inches and the air gap was 0.060 inches using 0.018 inch diameter YIG spheres. Solder joints were only used at the midpoints of the transitions between the half loops and the full loops. The performance of this combination deteriorated significantly in that the insertion loss climbed from less than 1 dB to greater than 6 dB at 13 GHz. Return loss went from less than 10 dB to approximately 0 dB. This failure further confirmed the Ferretec belief in the conventional wisdom that full loop coupling can only be used in the low frequency end of the passband and was another signpost pointing away from the path which eventually resulted in finding the invention.
The Ferretec structure with half-loop coupling and solder joints between loops was shipped in 1985.
In July of 1990 Watkins-Johnson announced a 10-stage band reject filter in Microwave Engineering Europe. This device used a larger number of YIG spheres than had been previously tried. A family of filters built around this concept eventually resulted including filters which cover passbands from 6-18 GHz, from 12-18 GHz and from 6-12 GHz. 40 dB notches with 25 MHz notch width for the passband from 6-18 GHz is available with this family of filters and 40-50 MHz notches for 6-12 and 12-18 GHz is also available.
In all the prior art described above, full coupling loops have never been successfully used in the higher end of the frequency range. Further, it has often been assumed in prior art approaches that a circular configuration of the YIG spheres was necessary so that all spheres would experience the same magnetic field intensity even though the circular configuration is not the most efficient configuration in terms of air gap area required. Probably the biggest problem facing YIG filter designers over the years has been how to handle the spurious (2,2,0) Walker mode. When a YIG sphere resonates, it has a useful 110 mode which can be used to create either a bandpass or band reject filter as described in the '674 patent discussed above. However, the YIG sphere also resonates at harmonic frequencies and these resonances create irksome spurious modes or "spurs" that create unwanted notches in the passband. The '674 patent teaches that nonreciprocal coupling can be used to reduce or eliminate the 220 spurious mode, but it also teaches that the nonreciprocal coupling is generated by the use of circular polarization. The circular polarization is achieved by offsetting the YIG spheres from the plane of the coupling loop to provide spatial orthogonality and coupling a shunt capacitor to the coupling loop to provide a 90 degree phase shift.
Unfortunately, the achievement of circular or elliptical polarization is difficult and unpredictable, and all the ways of doing it taught in the '674 patent were so complicated that nobody in the industry adopted these teachings.
Therefore, a need has arisen for a simple YIG notch filter and bandpass filter design that uses full loop coupling both at the high and low frequencies, and which need not necessarily have the YIG spheres arranged in a circular configuration, and which can achieve nonreciprocal coupling in a predictable manner so as to minimize the effects of the (2,2,0) Walker mode.